Nitrogen-implantation-induced transformation of iron to crystalline Fe16N2 in epitaxial iron films

Abstract
Crystalline phase Fe16 N2 has been found in nitrogen‐implanted epitaxial iron films on mirror‐polished (100) MgO substrates. Implantations of N+2 at an acceleration voltage of 140 keV transformed the bcc iron into a partially ordered nitrogen martensite with a body‐centered‐tetragonal (bct) structure. The amount of the ordered part was increased to some extent by an annealing treatment in vacuum at 150 °C for 2 hr. An implantation with a dose of 4×1016 N2/cm2 transformed 60% iron in a film into the bct martensite. The constituent of the ordered phase (Fe16 N2 ) in the bct martensite was 16 wt. %. After the annealing, the constituent increased to 24%. It is proposed that a formation of lower temperature and metastable compound such as Fe16 N2 can be made available through energetic ion implantation.